Résumé: To deal with hardware reliability requirements of critical real-time computer applications, fault-tolerance provisions have become a widely accepted practice. Failures of software executing on these computers is equally critical, and the extension of fault tolerance to software is therefore desirable but it needs to be implemented in a specific manner. Redundancy in fault-tolerant software requires programs that are deliberately different from the original ones which they are intended to back up. Error detection and rollback provisions must be as independent as possible of the software segments which they protect. The recovery block concept pioneered by Randell meets these requirements. Skeleton routines are presented that illustrate the application of the recovery block to real-time programs, particularly those dealing with navigation and attitude control. The concept is seen to be compatible with certain ad hoc fault-tolerance techniques that are currently employed. A technique for reliability analysis of the resulting software system is developed. While specific software failure data are unfortunately not yet available, the exercise of this reliability model with a range of hypothetical failure rates shows that a very appreciable desensitization of the overall computer performance to software errors is possible by use of these fault-tolerauce provisions. Economic factors of applying fault-tolerant software are discussed. Memory cost for the additional software segments is an obstacle at present but will probably shrink to insignificance in the future. A positive approach to the faulttolerant software concepts seems warranted. A program for research to more fully explore the capabilities and limitations is suggested.

Langue: Anglais
Index décimal 621 .Physique appliquée (électrotechnique, génie civil, génie mécanique, ingénierie appliquée, principes physiques en ingénierie)
Thème Informatique

Mots clés:
Reliability
Fault-tolerant software
Independent test and evaluation
Primary and backup modules
Reliability models